1922] Allen: Studies on Marine Plujtoplankton at La Jotla 339 



numbers. Roughly, it may be said to be the most important genus of 

 planktonic diatoms. 



Chaetoeeras curvisetum CI. — Frequently confused in our records 

 with Ch. debile CI. I have not found any satisfactory way to dis- 

 tinguish the two forms while counting. When possible, segregation 

 has been made according to Gran's (1908) suggestion that interspaces 

 between the cells are biconvex in Ch. curvisetum and rectangular in 

 Ch. debile. Sometimes one and sometimes the other is very abundant 

 and both are very commonly found in our waters. Segregation is 

 attempted merely because the records, though imperfect, may some- 

 time show some value. 



In September the species reached a maximum of 276.000 per liter 

 at 8 p.m. on the 14th. It was missing entirely from many catches 

 before the 11th but showed a very strong and even development of 

 production after that date until the maximum was passed, when the 

 numbers gradually decreased. Average production for the month was 

 32,089 per liter. These numbers refer to cells. On account of 

 curvature of the chain of cells exact enumeration was often impossible 

 but the approximation is usually close. This species was frequently 

 missing in catches in the other three months and it is quite evident 

 that its abundance in September was clue to a favorable combination 

 of environmental factors. 



Chaetoeeras debile CI. has a record of distribution and of appear- 

 ance markedly similar to that of Ch. ctirrisetum CI., and there is little 

 to say about it at present. Maximum production of 31,000 per liter 

 was reached at 8 p.m. on September 17. 



Chaetoeeras decipiens CI. was not always clearly identified. Shape 

 of cells and of interspaces varied greatly and the typical form was 

 rarely seen. Basal fusion of the setae was the characteristic relied 

 upon for recognition and even it was sometimes questionable. 

 Numerous gradations between the typical form and some which were 

 otherwise scarcely recognizable were often found on the same slide. 

 Representation in September as well as in other months was rather 

 irregular, the maximum. 27,200, being reached on September 26 at 

 8 p.m. in a poorly developed three day pulse. Ch. decipiens was 

 entirely missing through two-thirds of October and a large part of 

 November. When present in the last three months of the year 

 representation was light. 



Chaetoeeras didymum Ehr. was never very prominent in any 

 catches, although its identity was probably often overlooked. Its 



